1. Field of the Invention
The invention relates to the catalytic polymerization of ethylene alone, or with other .alpha.-olefins, with silica supported chromate ester catalysts.
2. Description of the Prior Art
U.S. Pat. Nos. 3,324,095; 3,324,101; 3,642,749; 3,704,287; 3,752,795; 3,847,957 and Canadian Pat. No. 907,592 disclose the use of supported chromate ester compounds as catalysts for the polymerization of ethylene, alone, or with other .alpha. -olefins. These chromate ester compounds include silyl chromate esters, hindered ditertiary polyalicyclic chromate esters, and phosphorus, titanium and tin containing chromate ester compounds, as well as chromyl halides which can also be called oxyhalides of chromium. These chromate ester compounds may be used on activated inorganic oxide supports and the supported chromate ester compounds can, optionally, be treated with various organometallic reducing agents. These reducing agents include the dialkyl aluminum alkoxide type reducing agents which are disclosed in U.S. Pat. No. 3,324,095 and in Canadian Pat. No. 907,592. As disclosed in U.S. Pat. No. 3,324,095, the alkoxide, which may include a phenoxide compound, can be prepared in-situ, in a solution of the catalyst system. This may be accomplished by the addition of an alcohol or phenol and an aluminum alkyl to the supported silyl chromate ester.
Ethylene polymers made with these supported and reduced chromate ester catalysts tend to have a relatively high molecular weight. One of the means which is commonly employed for measuring the relative molecular weight of a polymeric material is a melt index procedure as disclosed in ASTM Procedure D-1238 (measured at 190.degree. C., and reported as grams per 10 minutes). Polymers having a relatively high molecular weight have a relatively low melt index value. Thus, polymers with very high molecular weight values may have melt index values of 0, i.e., a no-flow melt index, to about 0.1 (under a 44 psi load on the ram of the melt index test device) or flow index values of about 1 to 20 (under a 440 psi load on the ram of the melt index test device). Higher melt index ranges of the order of about 0.5 to 1, with a corresponding flow index value of about 50 to 100 can be obtained with the conventional diethyl aluminum ethoxide treated chromate ester catalyst by increasing the polymerization temperature and/or the hydrogen concentration, i.e., the hydrogen to monomer ratio, in the reaction system at a given loading of chromium on the catalyst support.
The need for higher reaction temperatures leads to additional processing and energy consumption processes. Further, where the reaction is conducted in a fluid bed reactor, as disclosed in Canadian Pat. No. 971,700 and in U.S. Pat. Nos. 3,790,036; 3,790,550 and 3,876,602, the use of higher reaction temperatures can lead to the fusing of the resulting polymer into chunks which cannot be fluidized and which cannot be readily removed from the reactor. Further, the need for higher hydrogen concentrations in order to achieve these higher melt index values at a given level of total reactor pressure has disadvantage(s) in that it lowers catalyst productivity and leads to polymers of relatively small particle size which have poor fluidizing properties in a fluid bed reactor process.
The use of the diethyl aluminum phenoxide or pentaethyl dialuminum phenoxide [(C.sub.2 H.sub.5).sub.5 Al.sub.2 (OC.sub.6 H.sub.5)] type reducing agents with the silyl chromate catalyst also tends to produce an ethylene polymer having a melt index range of about 0.3 to 10. However, these products have a very high cyclohexane extractables content which make them unacceptable for use in various applications.
Although ethylene polymers made with a silyl chromate catalyst which has been reduced with a diethyl aluminum ethoxide type reducing agent have a relatively low cyclohexane extractables content, their relatively high molecular weight ranges tend to preclude their use in various types of molding applications which require the use of polymers having higher melt index values of the order of about 0.2 to 1.0.
A further disadvantage of using the supported silyl chromate esters, to date, is the fact that it requires a relatively long time to accomplish the deposition of the chromate ester on the support.
British Pat. No. 1,109,944 indicates that phenol type compounds may be added to Ziegler type catalysts to provide ethylene polymerization catalysts which are more highly active as catalysts and produce better yields of polymer than catalyst systems made without the additive.
Prior to the present invention, however, it has not been possible to provide a chromate ester based catalyst which can be prepared in a relatively speedy manner and which would readily provide ethylene polymers having relatively high melt index values and relatively low cyclohexane extractable contents without the need for high concentrations of hydrogen as a chain transfer agent, and/or the use of high reaction temperatures during the polymerization reaction.